Process of producing piezoelectric transducers



Jan. 16, 1951 w. L. CHERRY, JR 2,538,554

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INVENTOR.

Jan. 16, 1951 w. CHERRY, JR 2,538,554

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PROCESS OF PRODUCING PIEZOELECTRIC TRANSDUCERS Filed Aug 22, 1947 4Sheets-Sheet 4 $113.5 LORIGINAL POLARIZATION PRE-POLARIZED I IAPPRQXIMATE REGION Z g I PIEZO-AGTIVITY vs. FORMING VOLTAGE m 1 Lu '1 lI E I, 21 x 1 1 I o I I l l l l I I 200 400 600 800 I000 I200 I400 I600I800 BIAS VOLTAGE E I2- Fig.5

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H/s' AGENT Patented Jan. 16, 1951 PROCESS OF PRODUCING PIEZOELECTRICTRANSDUCERS Walfir L. Cherry, J r., Northbrook, Ill., assignor to ZenithRadio Corporation, a corporation of Illinois Application August 22,1947, Serial No. 770,163

(Cl. TIL-327) 6 Claims.

This invention relates to a process for making piezo-electrictransducers and to the piezoelectric transducers formed by that processand has for an object to provide such a process which is relativelysimple and easy to carry out and to provide a resulting product which isinexpensive and rugged and, although highly sensitive, is substantiallyunaffected by ordinarily encountered temperature and humidityconditions.

The features of the present invention which or believed to be novel areset forth with particularity in the appended claims. The presentinvention itself, both as to its organization and manner of operation,together with further objects and advantages thereof may best beunderstood by reference to the following description taken in connectionwith accompanying drawings in which:

Figure 1 illustrates one embodiment of the product of my invention, and

Figures 2 through 6 illustrate certain characteristics of the productobserved during and after the process of making the product.

In Figure 1, a body I, which is a solid polycrystalline aggregate, isprovided on opposite faces with electrodes 2 and 3 to which suitableconductors 4 and 5 are connected. Application of pressure to the body inany direction which induces a change in the dimension of the body lyingbetween the electrodes causes the formation of an electric Charge on thetwo electrodes, so that the charge influences an externally connectedcircuit through the conductors 4 and 5.

-Such force may be applied to the body by a phonograph needle which runsin the groove of a phonograph record, so that alternating voltagesdeveloped between the conductors 4 and 5 accurately represent soundwhich was originally used to iormundulations in the groove on suchrecord.

The solid polycrystalline aggregate which forms the body I is formed ofindividual crystals bonded together, the crystalline material of whichhasa transition temperature above which, it is believed by reason ofsymmetry, the individual crystals are not piezo-electric but below whichtransition temperature such crystals are piezo-electric. Theseindividual crystals in the solid polycrystalline aggregate are formedand then caused to become piezo-electric in the aggregate by thefollowing process which is believed at present to cause thepiezo-electric axes of individual crystals to assume a preferred commondirection.

One such specific polycrystalline aggregate body which was found to behighly piezo-electric and permanent in nature is formed as follows:

Barium titanate, thoroughly mixed with small amount of a glass formingoxide, such, for example, as silica with possible additions of alumina,magnesia, or calcium oxide, is brought to a sufiiciently hightemperature to bond the aggregate into a solid body which may be of anydesired form.

While glass-forming oxides have been men tioned as such a bonding agent,and are preferred because their use as the bonding agent renders theresulting body substantially impervious to normally encounteredtemperatures and humidity conditions, any bonding agent may be utilizedwhich has high electrical resistance under the desired workingconditions and which is sufilciently rigid to maintain individualcrystalsof barium titanate substantially immobile. At the same time thebonding agent should occupy a sufilciently small space that thepiezo-electric effect is not reduced by the presence of inert materials.An example of another suitable bonding agent for certain purposes isphenolformaldehyde and other similar resins.

Another example of a suitable polycrystalline aggregate body is asfollows: Barium strontium titanate mixed thoroughly with a suitablebonding agent as previously described, is brought to a sufflciently hightemperature to bond the aggregate into a solid body of desired form.

Electrodes are formed on two opposite faces of the body. Theseelectrodes may be of any suitable conducting material such as metal foilor paint. Good results have been secured with electrodes of silver paintbaked on at a suitably elevated temperature. Conductors 4 and 5 areattached to the electrodes securely, as by baking these on at theconduction point or soldering them thereto.

With the device thus constructed a unidirectional potential having amagnitude preferably suificient to produce a field of at least 2500volts per centimeter in the body is applied between the two electrodes 2and 3 through conductors 4 and 5 for a time sumcient to attainasubstantial amount of permanent piezo-electric effect in they body. Goodresults have been obtained both at normal room or ambient temperaturesand at elevated temperatures which, however, must be lower than theabove mentioned transition temperature. Because of the convenience, itis now preferred to produce this permanent piezoelectric effect atnormally encountered room temperatures.

Experience indicates that the continuous potential applied should bepreferably as large as possible, provided, however. that it is not solarge as to cause the body to become so substantially semi-conducting asto decrease or destroy its usefulness. Such action appears to occur byactual dielectric breakdown or sparking. For the barium titanateaggregates. it has been found that a voltage is suitable which producesa field as high as 25,000 volts per centimeter in the body.

In Figure 2, the following characteristics are shown graphically forsuch a body whose thickness is about 0.4 millimeter and whose materialis a bonded aggregate of barium titanate. The percentage of final outputis plotted as ordinate and the time for which a forming voltage of 340volts was applied is plotted as abscissa on a semilogarithmic scale. Theupper curve It) indicates such output measured in the presence of theforming voltage, while the lower curve ii indicates such output voltagemeasured one minute after removal of the forming voltage.

In these data all actual readings of output" were taken by finding theaudio output voltage with a mechanical force of known magnitude appliedat an audio frequency rate, from which the mechano-electrical couplingfactor K is readily determined. This coupling factor K" is equal to thesquare root of the inverse ratio of mechanical energy per cycle ofapplied vibration to electrical energy per cycle of alternating currentenergy thus produced.

It is evident from both curves I and II that the voltage output of thebodies for a given applied stress increased .over a substantial periodof time until a rather well defined saturation was produced whichoccurred only after a voltage was applied continuously for from about5000 to 20,000 seconds, or from about one and one half to eight hours.

Figure 3 shows similarly plotted curves l2 and I3 when 510 volts wasapplied continuously to similar bodies. These curves show that similarsaturation occurred in somewhat less time (about 7000 seconds, or about2 hours).

Figure 4 shows similarly plotted curves I4 and i5 when 980 volts wasapplied continuously to similar bodies. These curves again show thatsimilar saturation occurred in still less time (about 600 seconds, orabout minutes).

It is therefore evident that the observed saturation in the resultingpermanent piezo-electrio effect is produced in less time as thepolarizing voltage is increased, so long as the polarizing voltage isnot so high as to break down the insulating qualities of the bodyseverely and cause it to become conductive by dielectric breakdown orsparking.

Figure 5 shows a composite of such results in such a way as to indicatethat there is a rather well defined minimum polarizing voltage, about100 volts (2500 volts per centimeter), below which rapidly decreasingpermanent piezo-electric effects are produced, and that at maximumusable voltages the best permanent effects are obtained. In the curve ISin Figure 5 the actual coupling factor K, as defined above, measuredfive minutes after removal of the forming voltage is plotted as ordinateagainst the amount of forming or bias voltage plotted as abscissa, eachbias. voltage having been applied long enough to reach the saturationdescribed above.

.It is evident from this curve It that better permanent piezo-electriceffects are obtained with larger bias voltages applied for the properlength of time.

It should be noticed that the piezo-electric effeet thus producedcontinues to decrease longer than five minutes after the bias voltage isremoved, but that all rapid changes are complete within five minutes,and especially that changes taking place later are always a decrease inpiezoelectric eifect over a long period which is substantiallyproportioned to the first activity in all cases.

The curve It was made from measurements made on the same bodiessubjected first to low forming voltages and later to higher voltages. Inorder to determine the effect of prior electrical history of thebodiifi, new readings were made, which are shown plotted as curve H inFigure 5, in which newly made bodies were used for each point of thecurve. In other words, a body polarized at 1000 volts for a point on thecurve at the 1000 volt abscissa had never previously been polarized atany other voltage. This curve shows that such previously unpolarizedbodies produce substantially more output, or in other words, are more"active," when polarized at voltages above about 500 volts (12,000 voltsper centimeter) and become quite highly active when polarized at 1000volts (about 25,000 volts per centimeter).

It is therefore preferred to polarize polycrystalline aggregate bodieswhich have never before been polarized at about 25,000 volts percentimeter for at least the saturation time, in order to obtain themaximum possible permanent piezo-electric effect, although somewhat lessactivity is secured at lower voltages and with bodies which have somesignificant electrical his- It is, in fact, preferred in order torealize the maximum permanence of the piezo-electric effect to form thebodies in such a manner for about 20 hours.

In applying the forming voltage to these bodies, it has been foundconvenient, since their internal resistances when active are very high,to supply the voltage from dry batteries through a high resistance of,for example, about ten megohms. The voltages indicated are, however,always measured across the electrodes of the body. In so applyingpolarizing voltage is has been observed that some of the bodies whichbreak down internally and have therefore relatively low internalresistance are of little use, and that others which retain the highestinternal resistance are somewhat less active than those whose internalresistance drops somewhat during the forming period. This phenomenon isnot fully understood and is set forth here merely as an aid in themanufacture of the devices.

As an indication of the permanence of bodies so constructed,measurements were made of output for about three weeks, afterpolarization, the bodies being maintained at a temperature of degrees C.The results of such measurements are shown in Figure 6 in whichmechano-electrical coupling coeflicient K is plotted as ordinate againsttime as abscissa.

In Figure 6, curve It shows the decrease in activity as a function ofelapsed time since polarization, and indicates that no substantialdecrease in activity occurs subsequent to the fourth day followingpolarization. This curve incor porates data taken from observation madeof bodies whose internal resistance dropped somewhat duringpolarization.

Curve 20 is similar to curve l0, but was made from measurements ofbodies whose internal re- Thus it may b seen that the activity of bodieswhose internal resistance dropped somewhat during polarizationisconsiderably higher than that of bodies whose internal resistanceremained high; in both cases, the decrease in activity approaches anasymptotic value within the first week following polarization.

It has been found by experimentation that bodies maintained at roomtemperature during the aging period approach substantially the sameasymptotic value, but at a much slower rate.

The artificial polycrystalline aggregate disclosed herein hascharacteristics distinguishable from those of natural crystals such asRochelle salt or quartz crystals in that such natural crystals have afinite number of planes of symmetry around the electrical axis whereaswith the presently disclosed material there is a common axis ofcylindrical symmetry with respect to its mechanical and electricalproperties and there are an infinite number of planes of symmetry.

While the piezo-electric effect which appears in bodies formed inaccordance with this invention is by no means fully understood, it isthought that the individual crystals in the polycrystalline aggregateare of a form which is symmetrical above transitiontemperature, andasymmetrical about their centers below that temperature, and that theapplication of a continuous electric field through such aggregate for asumciently long time tends to orient the slight asymmetry of thecrystals so that their electric axes become most nearly parallel to thedirection of the applied uni directional field, although the individualcrystals evidently do not move in the aggregate.

It is important to note that the direction and sense of theunidirectional applied electro-static field must remain the samethroughout the proces of formation, and, asexplained above, it ispolarized or formed to the saturation point.

While particular embodiments of the present invention have been shownand described and particular processes explained, it will be obvious tothose skilled in the art that changes and modifications may be madewithout departing from this invention in its broader aspects and,therefore, the aim in the appended claims is to cover all such changesand modifications as fall within the true spirit and scope of thisinvention.

I claim:

1. The process of producing a permanent piezoelectric eflfect in a solidpolycrystalline aggregate of crystals individually possessingpiezo-electric properties below a predetermined transition temperatureand losing said properties above said transition temperature, whichprocess comprises: maintaining said aggregate at a temperature belowsaid transition temperature; producing a unidirectional electrostaticpolarizing field in said aggregate; and maintaining said polarizingfield for a substantial time at least approaching that required forsaturation of said effect.

2. The process of producing a permanent piezoelectric effect in a solidpolycrystalline aggregate of crystals individually possessingpiezo-electric properties below a predetermined transition temperatureand losing'said properties above said transition temperature, whichprocess comprises: maintaining said aggegate at a temperature below saidtransition temperature; producing in said aggregate a unidirectionalelectrostatic polarizing field of a value between about 2500 volts percentimeter and the breakdown value for said aggregate; and maintainingsaid polarizing field for a substantial time at least approaching thatrequired for saturation of said eflect.

3. The process of producing a permanent piezoelectric efiect in a solidpolycrystalline aggregate of crystals individually possessingpiezo-electric properties below a predetermined transition temperatureand losing said properties above said transition temperature, whichprocess comprises: maintaining said aggregate at a temperature belowsaid transition temperature; producing in said aggregate aunidirectional electrostatic polarizing field of the order of 15,000volts per centimeter; and maintaining said polarizing field forasubstantial time at least approaching that required for saturation ofsaid effect.

4. The process of producing a permanent piezoelectric efiect in a solidpolycrystalline aggregate of crystals individually possessingpiezo-electric properties below a predetermined transition temperatureand losing said properties above said transition temperature, whichprocess comprises: maintaining said aggregate at a temperature belowsaid transition temperature; producing in said aggregate aunidirectional electrostatic polarizing field oi the order of 25,000volts per centimeter; and maintaining said polarizing field for asubstantial time at least approaching that required for saturation ofsaid effect.

5. The process of producing a permanent piezoelectric eflect in a solidpolycrystalline aggregate of crystals individually possessingpiezo-electric properties below a predetermined transition tem peratureand losing said properties above said transition temperature, whichprocess comprises:

maintaining said aggregate at a temperature be- 10w said transitiontemperature; without previous application of any electric field,producing a unidirectional electrostatic polarizing field in saidaggregate; and maintaining said polarizing field for a substantial timeat least approaching that required for saturation of said effect.

6. The process of producing a permanent piezoelectric efiect in a solidpolycrystalline aggregate of crystals individually possessingpiezo-electric properties below a predetermined transition temperatureand losing said properties above said transition temperature, whichprocess comprises: maintaining said aggregate at normal roomtemperature; producing a unidirectional electrostatic polarizing fieldin said aggregate; and

maintaining said polarizing field for a substantial time at leastapproaching that required for saturation of said effect.

WALTER L. CHERRY, JR.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,798,101 Nicolson Mar. 24, 19311,886,234 Meissner Nov. 1, 1932 1,969,379 Meissner Aug. '7, 19342,235,489 Rath Mar. 18, 1941 2,260,842 Schwarzhaupt Oct. 28, 19412,402,515 Wainer June 18, 1946 2,424,111 Navias July 15, 1947 (Otherreferences on following page) FOREIGN PATENTs DeBrettevflle, Jr. Phy.Rec-69: 687, June 1946.- Number Country t WeinerPeper, 89th MeetingElectrochemical 302,726 Great Britain Mar. 14, 1930 ;Y-% 8 }g.A1a.,tAprl1 11. 1946.

' a y, ezoelec rlclty," McGraw-Hlll, OTHER REFERENCES 5 pp. 4, 198,233,234. 235, 260, 261, 614. 1946, New W111 and Golgmen, Compt. Rend. Acad.801. York. (URSS) 46: No. 4, 1939, 1945, Moscow; 49: Shepard Roberts,Physical Review, v01. 71, No. 177-480, Oct. 30, 1945, Moscow. 12. pages890-895, June 15, 1947.

Coursey and others, Nature 156: 490,717, 1945; Donley, R. C. A. Review,vol. VIII, No. 3, pp. 157: 297-298, March 9, 1946. 10 539-553, Sept.1947.

Disclaimer '2,538,554.-Walter L. Cherry, Jr., Northbrook, Ill. Processor Pnonucmc PIEZOELEUIRIC TRANSDUCERS. Patent dated J an. 16, 1951.Disclaimer filed Dec. 22, 1951, by the inventor and the assignee, ZenithRadio C'orporatz'on.

Hereby enter this disclaimer to claims 1 to 6 inclusire of said patent.

[Oficial Gazette February 19, 19.52.]

1. THE PROCESS OF PRODUCING A PERMANENT PIEZOELECTRIC EFFECT IN A SOLIDPOLYCRYSTALLINE AGGREGATE OF CRYSTALS INDIVIDUALLY POSSESSINGPIEZO-ELECTRIC PROPERTIES BELOW A PREDETERMINED TRANSITION TEMPERATUREAND LOSING SAID PROPERTIES ABOVE SAID TRANSITION TEMPERATURE, WHICHPROCESS COMPRISES: MAINTAINING SAID AGGREGATED AT A TEMPERATURE BELOWSAID TRANSITION TEMPERATURE; PRODUCING A UNI DIRECTIONAL ELECTROSTATICPOLARIZING FIELD IN SAID AGGREGATE; AND MAINTAINING SAID POLARIZINGFIELD FOR A SUBSTANTIAL TIME AT LEAST APPROACHING THAT REQUIRED FORSATURATION OF SAID EFFECT.